The anode and the cathode of such an electrolytic cell are immersed in an electrolytic solution when used in various electrolytic processes. They can be used in various arrangements: for example, they may be separate from each other in a diaphragmless electrolytic cell, they may be placed on both sides of a diaphragm or an ion-exchange membrane away from the diaphragm or membrane, they may be used in a finite-gap electrolytic cell, in which the two electrodes are placed on both sides of a diaphragm or an ion-exchange membrane with a minimum distance from the diaphragm or membrane, or they may be used in a zero-gap electrolytic cell, in which an ion-exchange membrane is sandwiched between the two electrodes with no space left. In all of such cases, the side of the anode and the cathode that faces the diaphragm or ion-exchange membrane is the site for the main reaction and is defined as the front, while the other side the back.
Electrolytic electrodes for ion-exchange membrane electrolysis, in particular, the anode and the cathode of a finite- or a zero-gap electrolytic cell, are produced using a conductive electrode substrate with a plurality of holes that is expanded mesh, a punched perforated plate, or wire netting or an object having a similar shape. The ordinary method for producing such an anode and a cathode includes intentionally forming an electrode catalyst layer on either side, or the front, of two faces of the conductive electrode substrate with a plurality of holes and placing the electrode substrates on both sides of an ion-exchange membrane with their front facing the membrane with no or only a minimum space provided.
For electrolytic sodium hydroxide production, researchers have proposed many different ion-exchange membrane alkali chloride electrolytic cells that can produce high-purity alkali metal hydroxides at high current efficiency and low voltage, in particular, filter-press zero-gap electrolytic cells, in which the ion-exchange membrane is sandwiched between the anode and the cathode with no space left. A filter-press zero-gap electrolytic cell is composed of many bipolar structures arranged with cation-exchange membranes therebetween, and each bipolar structure has an anode chamber and a cathode chamber positioned with their back facing each other. The cathode chamber contains a hydrogen-producing cathode that is in contact with the cation-exchange membrane, and the anode chamber contains a chlorine-producing anode that is in contact with the other side of the cation-exchange membrane.
The substrate of the anode of this kind of electrolytic cell is usually made of a titanium-based material, and that of the cathode is usually made of nickel or a nickel alloy. The anode and the cathode are both produced using a conductive electrode substrate with a plurality of holes that is expanded mesh, a punched perforated plate, or wire netting or an object that has a similar shape (hereinafter also collectively referred to as a conductive substrate with a plurality of holes). One side of such a substrate is coated with an electrode catalyst layer that contains an electrode catalyst component composed of an expensive and rare platinum-group metal and/or its oxide (hereinafter, also referred to as a platinum-group metal or the like), and this side of the electrode is used for the main reaction and defined as the electrode's front.
Patent Literature 1 discloses a method for manufacturing an electrolytic electrode for zero-gap electrolytic cells, and a category of the electrolytic cells is the one in which an ion-exchange membrane is sandwiched between the anode and the cathode with no space left. The publication specifies, for example, the thickness and the open area ratio of the conductive substrate with a plurality of holes, the thickness of the electrode catalyst layer, and the surface roughness of the electrode for each of an anode and a cathode, and also mentions pretreatments such as annealing, shaping, flattening by rolling, roughening by blasting, washing and etching with an acid, and corrosion resistance enhancement.
In known manufacturing methods, usually, a conductive substrate with a plurality of holes that has the aforementioned shape is subjected to pretreatments such as annealing, shaping, flattening by rolling, roughening by blasting, washing and etching with an acid, and corrosion resistance enhancement, and then its front is coated with an electrode catalyst layer that contains an electrode catalyst component composed of an expensive platinum-group metal or the like. The step of forming the electrode catalyst layer is referred to as activation step, and the activation step usually includes three steps: applying a coating solution that contains a starting material from which the electrode catalyst component can be derived (hereinafter also referred to as the starting material) to the substrate and then drying and firing the obtained coating layer. More specifically, in a typical activation step, a coating solution prepared by dissolving the starting material is applied to the front of the conductive substrates with a plurality of holes after pretreatments such as those mentioned above, and then the obtained coating layer is dried and fired to form an electrode catalyst layer. The electrode catalyst layer is then grown to the desired thickness by repeating the three operations, i.e., application, drying, and firing, several times until as much of the electrode catalyst component as required adheres to the front of the conductive electrode substrate. In this way, the electrode catalyst layer that contains an electrode catalyst component composed of an expensive platinum-group metal or the like (hereinafter also referred to as the catalyst layer substance) is formed. Usually, the coating solution is applied to the substrate by means such as spraying, brushing, and electrostatic coating, and the dried coating layer is fired by heating usually in an electric furnace or similar devices.